In a gas turbine, fuel nozzles are used to mix a compressed working fluid such as air with a fuel for combustion within a combustion zone that is positioned downstream from the fuel nozzles. Some fuel nozzles are configured to operate on one type of fuel such as a gas fuel during gas fuel operation mode and on a second type of fuel such as a liquid fuel during liquid fuel operation mode. Where a liquid fuel is used for diffusion mode operation, coking forms and high thermal stresses occur at a central body tip portion of the fuel nozzle.
A particular fuel nozzle configuration that is capable of both gas and liquid mode operation generally includes an outer premix flow passage that is at least partially defined by a burner tube and a plurality of swirler vanes that extend radially inward from the burner tube into the premix flow path. The fuel nozzle further includes a central body that is coaxially aligned with the burner tube and that extends at least partially through the burner tube. The central body defines a recirculation zone for flame stabilization in a central area of a combustion zone that is downstream from the fuel nozzle. A liquid fuel cartridge (LFC) extends at least partially through the central body and/or the burner tube and/or the premix flow passage. The LFC being coaxially aligned with the burner tube. A LFC tip assembly is at least partially disposed within the center body. The LFC tip assembly includes a liquid fuel injector that is disposed at a downstream end of the center body and generally adjacent to an outlet of the premix flow passage. The liquid fuel injector tip is generally adjacent to the combustion zone of the combustor.
The LFC tip and/or the LFC tip assembly provide a mechanism for generating a flame using a liquid fuel during startup and operation of the combustor. However, because of the proximity of the LFC tip with respect to the combustion zone of the combustor, heat from the combustion gases and/or heat due to heating during premix mode operation may damage the center body tip over time. In addition, in certain configurations where the fuel nozzle is configured to flow both the gas fuel and the liquid fuel, the liquid fuel causes coke deposit formation or coking on a heated downstream surfaces of the center body tip. Current designs use a curtain of air that is routed through the center body to cool the center body tip. However, despite the general effectiveness of this system and/or method for cooling the nozzle tip, center body tips are still a life limiting component of this type of fuel nozzle. Therefore, a center body tip assembly with an improved system and/or method for cooling the center body tip would be useful in the industry.